The present invention relates to an improvement in a catalytic dehydrogenation process and in a reactor used therein. It relates particularly to a method whereby the amount of undesirable nonselective cracking promoted by active metal surfaces in a catalytic dehydrogenation reactor can be substantially reduced.
In the catalytic dehydrogenation of a hydrocarbon having an alkyl moiety to a corresponding alkenyl compound, for example, the conversion of ethylbenzene to styrene, ethyltoluene to vinyltoluene, and butene to butadiene, there is also a certain amount of cracking which results both from exposure of the hydrocarbon feed to elevated process temperatures whereby some thermal cracking takes place and from contact of the hydrocarbon with hot metal surfaces within the reactor prior to contact with the catalyst where those metal surfaces catalyze a low yield conversion or decomposition of the feed. U.S. Pat. No. 3,474,153 documents the use of preferred metals of construction typically low in nickel content for minimizing such yield loss at reaction conditions. Both the strictly thermal cracking and the cracking which is promoted by an active metal surface can begin at a temperature as low as 450.degree. C and since catalytic dehydrogenation process temperatures range generally from about 550.degree. C to about 650.degree. C, it is obvious that loss of hydrocarbon feed to these destructive side reactions can be significant. Certain stainless steel alloys and some other such corrosion and heat resistant alloys commonly used in constructing catalytic dehydrogenation reactors actively promote nonselective cracking and alloys containing a high proportion of nickel are particularly bad in this respect. Both of these side reactions can detract substantially from the ultimate yield of the desired olefinic product.
The first of these destructive side reactions can be minimized by reducing the residence time of the hydrocarbon feed within the void space in a cracking reactor so that the feed contacts the catalyst more rapidly after reaching a cracking temperature. This can be accomplished by changes in reactor design. For example, in the newer radial dehydrogenation reactors, the feed is introduced into the central void of a hollow cylindrical catalyst bed and it passes from there outwardly through the surrounding cylindrical body of catalyst to the outlet. In the copending application of Sutherland, Ser. No. 760,896, filed Jan. 21, 1977, in the hands of a common assignee, the volume of the central void in such a reactor, and consequently the residence time of the incoming feed within that void is reduced either by shaping the catalyst bed into a truncated hollow cone with the larger end toward the feed inlet or by introducing a generally conically-shaped filler into the central void. In either case, the residence time of the feed within the central void is reduced without changing the contact time in the catalyst bed. The total metal surface which the feed has to contact before reaching the catalyst bed is not greatly reduced by the first of these expedients and the metal surface area is substantially increased by the second. Thus, the problem of nonselective conversion caused by an active metal surface remains.